The most conventional date timepieces are described in particular in the work entitled “Les montres compliquées” (Complicated Watches) by Franøois Lecoultre and edited by Editions Horlogères in Bienne.
In an instantaneous date mechanism, star wheels for the days of the week, the month of the year and the date are all activated by the same perpetual lever, which is pivotally mounted relative to the plate or to a bridge of the timepiece movement, and which accomplishes its date change function when the day changes, in an abrupt movement, at a precise moment, by the action of a beak and two clicks, comprised in said perpetual lever. This jump is performed around midnight.
This perpetual lever is returned by a spring to a rest position where one of the beaks thereof abuts on a first sector of a month cam, carried by a month star wheel with 12 teeth which completes one revolution per year. The radius of this month cam is representative of the number of days in the month concerned, either in the form of a notch of greater or lesser depth, or in the form of a projecting portion of greater or lesser extension.
This perpetual lever is abruptly pivoted by a wheel, which is connected to the movement and completes one revolution in 24 hours, said wheel carrying a drive pin for a heart piece against a roller mounted on a pivoting part returned by a spring. When the heart piece passes a point, this starts the abrupt movement of an index which drives a beak of the perpetual lever.
During its pivoting movement, the perpetual lever moves away from said month cam, in order to return to abut on the month cam at the end of its movement, or to the same position, if it occurs during the month, or to abut on the sector next to the first sector of said month cam, if said cam has pivoted when actuated on the occasion of a change of date.
Another beak of the perpetual lever controls the pivoting of a day of the week star wheel. Since the sequence of days is perpetual, no particular mechanism is required, since it is sufficient to increment the position of this star wheel by one step.
The pivoting of a thirty-one star wheel with thirty-one teeth is achieved via a first thirty-one click, which is pivotally mounted on the perpetual lever and whose pivoting is limited by a pin fixed to said lever. This thirty-one star wheel pivots integrally with a first cam and a second cam both of which are snails.
The first snail cam is arranged to cooperate with a first feeler spindle comprised in a pivoting month lever, returned towards said first cam by a spring. The function of this first cam is to cause the month lever to fall at the appropriate time. The month lever includes for this purpose, opposite the first feeler spindle, a second feeler spindle arranged to cooperate with the month star wheel with 12 teeth, and to push said month star by one tooth when the first feeler spindle falls from the large radius to the small radius of the snail.
The second snail cam includes a simple catch, against which a second click acts, called the end of month make up click, carried by the perpetual lever, and which abuts thereon via a spring, and this second click only has an effect for months which do not have 31 days.
At the end of a February with 28 days, the beak of the perpetual lever is in the deepest notch of the month cam, and, during the change from the 27th to the 28th of the month, the second click is engaged in the catch of the second snail cam. This particular adjustment of the perpetual lever allows the second click to drive four teeth of the thirty-one star wheel together, when the perpetual lever jumps around midnight on the 28th of the month. Simultaneously, the first snail cam actuates the pivoting of the month lever, which drives the month star wheel, and thus the month cam, to pass to the sector for the next month, which in this case is March. Consequently, the perpetual lever takes a new rest position during the month of March, which is such that the fall of the second click is delayed by four days. Thus, from the 28th March to the 1st April, this second click only drives one tooth of the thirty-one star wheel at a time.
Likewise, the notch of the month cam for the months of 30 days is arranged so that the second click drives two teeth of the thirty-one star wheel on the evening of the 30th of the month.
Likewise, the notch of the month cam for the months of February with 29 days in leap years is arranged so that the second click drives three teeth of the thirty-one star wheel on the evening of the 29th of the month.
This tried and tested system requires two clicks to ensure the proper operation of the instantaneous perpetual date mechanism. It is difficult to adjust the hands. This arrangement is vulnerable to any disturbance, connected to a shock or improper operation, and must be returned to the workshop if the positions of the parts are lost.
CH Patent Application No. 660 440 A3 in the name of DUBOIS & DEPRAZ SA discloses a perpetual date mechanism which includes a large lever which is pushed by the movement and includes five fingers and a click. This click abuts on the periphery of a snail cam fixed to a thirty-one day wheel. Said wheel carries an actuating finger able to cooperate with a forty-eight month wheel. A first finger, formed by one end of the lever, is located on the path of an actuating finger, which can be driven by a pin carried by the twenty-four hour wheel. This actuating finger carries a convex dorsal part forming a cam, which actuates said perpetual date mechanism. In leap years, a second finger enters into contact with a lever comprised in a leap year cam. A third finger cooperates with a twelve month cam. A fourth finger forming a drive beak cooperates with the toothing of a thirty-one day wheel carrying the snail cam, said fourth finger works every day, whereas the click carried by the lever only works at the ends of the months. A fifth finger forming a second drive beak cooperates with a seven day star wheel. Due to this construction, disruptions are different depending on whether they occur for days with one jump (from 1st to 27th) or days with several jumps (28th to 31st).
Consequently the torque is used irregularly and the behaviour of the mechanism differs according to the duration of the current month.